1. Field of the Invention
This invention relates to a method of mounting electronic parts on the predetermined positions of a printed circuit board, and more particularly to a method of aligning the center of the electronic parts supplied in a jig with that of a suction tube of an automatic chip-mounting apparatus.
2. Description of the Prior Art
Recently, various kinds of automatic chip-mounting apparatuses have been used for automatically positioning miniaturized electronic parts of cylindrical shape, plate-like shape, disc-like shape or the like, such as, for example, resistors, capacitors or the like (hereinafter referred to as "chips") on a printed circuit board and fixedly mounting such chips thereon.
Generally, the automatic chip-mounting apparatus includes a jig having a plurality of cavities in which chips are charged. The cavities are arranged in alignment with positions of the printed circuit board on which the chips are to be mounted. The chips are fed to the cavities of the jig by a suitable means. The jig charged with the chips is shifted to a predetermined position where the chips are sucked up from the cavities onto a plurality of suction tubes provided at suction heads of the chip-mounting apparatus which are arranged in alignment with the cavities of the jig. The chips sucked up onto the suction tubes are transferred to the printed circuit board where suction is released so that the chips may fall to be precisely positioned on the printed circuit board corresponding to the arrangement of the chips in the cavities and securely adhered thereto by means of an adhesive applied onto the printed circuit board in advance. Then, the adhesive is subjected to dry and the printed circuit board having the chips mounted thereon to a soldering station so that the chips may fix on the board.
However, according to the mounting operations of the chips on the printed circuit board described above, it is extremely difficult to arrange the chips precisely on predetermined positions of the board. The chips are liable to be misregistered with respect to the printed circuit board ranging about 0.3 mm in the longitudinal direction (y-axial direction) and/or the lateral direction (x-axial direction) of the printed circuit board on a plane.
The misregistration is caused due to the inaccuracy of dimensions of individual chips. A chip has considerable variability in dimensions due to various factors occurring in the manufacturing process such as a dimensional error of the chip to be finished, a variation in dimensions of the chips caused by the work on the chip such as the coating on the chip, and the like. For example, a chip having basic dimensions of 1.25 mm.times.2.00 mm generally includes a dimensional error of about 0.25 mm in each direction. In order to compensate such a dimensional error of the chip, the cavities of the jig each must be formed to have a size sufficient to receive therein a chip having a maximum dimensional error of, for example, 0.25 mm and have a clearance of, for example, 0.25 mm defined around the chip received therein. In this example, the cavity must have a size of about 1.7-2.4 mm.times.2.4-2.5 mm.
Accordingly, the charging of the chip having basic dimensions of, for example, 1.25 mm.times.2.00 mm in the cavity creates a clearance of about 0.25 mm around the chip. When the chip has a dimensional error, the clearance is, of course, varied. This does not cause any problem when the chip is precisely positioned at the center of the cavity, however, the suction position of the chip is about 0.75 mm off in maximum when it is deviated from the precise position in the cavity.
On the other hand, the suction tubes of the suction head and the corresponding cavities of the jig can be mechanically designed with high accuracy to a degree sufficient to align the center of each of the suction tubes with that of the corresponding cavities on the same central line and carry out the mounting of the chips in a manner to permit the suction tube to descend to suck up the chips charged in the cavities, ascend while holding the chips thereon and then mount the chips on the predetermined position of the printed circuit board. Accordingly, when the chip is sucked up onto the suction tube at a position deviated from the predetermined or exact suction position thereof, the chip is mounted on a position of the printed circuit board deviated from the predetermined mounting position thereof resulting from the deviation of the chip in the cavity.
This misregistration of the chip is relatively easy to correct in an automatic chip-mounting apparatus of the single-mount type which is adapted to perform the sucking and mounting of chips onto the printed circuit board one by one, because the correction of the misregistration can be singly carried out. However, an automatic chip-mounting apparatus of the multi-mount type to which the present invention is directed renders the correction of the misregistration substantially impossible during the mounting operation, because it is constructed in a manner such that a plurality of suction tubes provided at the suction head such as, for example, vacuum tubings have to concurrently suck up a plurality of chips, for example, several tens to several hundreds of chips, at a time from a plurality of the cavities provided at the jig and mount them on the predetermined various positions of the printed circuit board.
Thus, the misregistration of the chip can not be eliminated by the conventional automatic chip-mounting apparatus of the multi-mount type. Accordingly, it has been attempted to decrease the misregistration by increasing the dimensional accuracy of individual chips so as to minimize the clearance of each cavity. However, this is not effective to miniaturized chips.